This invention relates to image sensors, and more particularly to an image sensor comprising a plurality of photo-electric conversion elements having a "sandwich structure", and to a method of manufacturing such an image sensor.
Recently, a close contact type image sensor having a reading unit with a length equal to the width of an original document has been developed for use as a photo-electric converter in the image input section of a fascimile machine or the like. Use of such an image sensor eliminates the need for a reducing optical system, and the image sensor itself can be made small in size. As a result, a variety of close contact type image sensors are available in the art.
Conventional close contact type image sensors employ a line of photo-electric conversion elements, each having a so-called "sandwich structure" which is made up of (1) a metal electrode, (2), a translucent electrode and (3) a photo-electric conversion layer of amorphous silicon (a-Si:H) interposed between the two electrodes. In this type image sensor the translucent electrodes of the photo-electric conversion elements generally are common; that is, one common translucent electrode is used for all of the photo-electric conversion elements. This is for convenience in manufacture.
Such close contact type image sensors may be manufactured by laminating a metal electrode, an amorphous silicon layer, i.e., a photo-electric conversion layer, and an indium tin oxide (ITO) layer, i.e., a translucent electrode, all in the stated order, on a substrate. The upper layer, being affected by the lower layer, has less dimensional accuracy. Accordingly, it has been found desirable to use split metal electrodes for greater dimensional accuracy on the lower layer side, and to form the translucent electrodes on the upper layer side as a common electrode.
On the other hand, such an image sensor may be manufactured by laminating a translucent electrode, a photo-electric conversion layer, and a metal electrode, all in the stated order, on a substrate, and splitting the translucent electrode to achieve greater dimensional accuracy. However, in this case, it has not been possible to successfully laminate a transparent, electrically-conductive film, such as indium tin oxide (ITO), to form the translucent electrode and the photo-electric conversion layer. Accordingly, prior to the formation of the photo-electric conversion layer, the translucent electrode is shaped (or patterned) by photolithographic etching. However, the etching operation may form residues, which make the junction surface of the photo-electric conversion layer and the translucent electrode unsatisfactory and results in unsatisfactory characteristics. Accordingly, even in the above-described structure of the image sensor, one common translucent electrode is used for all of the photo-electric conversion elements.
In the foregoing "sandwich structure" photo-electric conversion elements, Schottky junctions are formed between the photoelectric conversion layers and the translucent electrodes. Therefore, in order for each of the photo-electric conversion elements to have a satisfactory diode characteristic, a bias voltage may be applied to the photo-electric conversion element in such a manner that the translucent electrode has a negative potential. Accordingly, in operation, a negative bias voltage is applied to the translucent electrode which is the common electrode for all of the photo-electric conversion elements of the image sensor.
However, a disadvantage of the above-described image sensor is that, when it is required to use it with other devices, the image sensor does not have the desired degree of freedom in circuit design. This is disadvantageous in forming the image sensor and other elements on one and the same substrate.
Therefore, there are various ways of forming an image sensor in which the metal electrode, the photo-electric conversion layer and the translucent electrode are piled on the substrate and wherein the translucent electrode is split into individual electrodes necessary for the respective photo-electric conversion elements. However, such an image sensor is still disadvantageous, in that the characteristics of the bits are not uniform, and the ratio of light current to dark current in each of the photo-electric conversion elements cannot be made sufficiently large. Thus, it is difficult to use such image sensors commercially.